To date, mercury batteries have been widely used as a compact, high capacity power source for various portable electronic appliances, particularly for medical ones. However, the use of mercury batteries has been regulated in many countries due to the recent environmental problems caused by heavy metal pollution. As an alternative battery system, zinc-air batteries have been developed. This battery system uses oxygen in the air as a cathode depolarizer instead of mercury oxide used in mercury batteries. The above replacement enables a reduction in the required amount of mercury while maintaining almost the same operating voltage per unit cell. Furthermore, for a cell of the same size, the zinc-air battery is approximately 40% lighter than the mercury battery while it has a two times higher discharge capacity. The button or coin type of zinc-air flat type cells are being used as a single cell in hearing aids and pagers, and recently the demand has been rapidly increasing. FIG. 4 illustrates a cross sectional view of a typical zinc-air button type cell, whereas 30 indicates a gelled anode consisting of amalgamated zinc powder of 3% or less mercury content, viscous gelling agent, and an alkaline electrolyte. The alkaline electrolyte is a potassium hydroxide aqueous solution in which zinc oxide is dissolved. 31 in FIG. 4 denotes an anode cap serving also as a negative terminal, while 32 and 33 denotes a gasket and a cell can, respectively. At the bottom of the cell can, at least one air vent, denoted 37 in FIG. 4, is provided to serve as a positive terminal. The space between the cell can 33 and the anode cap is hermetically sealed with the gasket 32 by pressing and curling the upper flange of the cell can. The hydrophobic membrane 36, the air electrode 35 of cathode, and the separator 34 are layered in order at the inner bottom of the cell can 33, the periphery of which is clamped with the gasket 32 to maintain sealing. A diffusion paper 38 is held in the gap between the hydrophobic membrane 36 and the central area of the bottom of the cell can 33. The hydrophobic membrane 36, generally made of microporous polytetrafluoroethylene (PTFE), prevents electrolyte leakage from inside of the cell. The diffusion paper 38 helps distribute air entering from the air vent 37 during discharge uniformly on the air electrode 35 through the hydrophobic membrane 36. Normally, a sealing tape 39 is attached on the external bottom of the cell can 33 and keeps closing the air vent 37 to prevent deterioration until the battery is put into use.
Besides the single cell described above, recently a battery pack consisting of a plurality of single cells connected electrically to each other has been widely used as a power source for medical portable electronic appliances instead of mercury batteries. FIG. 5(A) and (B) depict the conventional structure of a battery pack in which 6 gas depolarizable button cells are connected in a series. The above structure is typically used in the zinc-air battery system described in U.S. Pat. No. 4,547,438. FIG. 5(A) shows an external side view of the existing battery pack, where 40 denotes a plastic container equipped with several ventilators 50. 41 indicates a plastic cover that composes a battery housing with the container 40. FIG. 5 (B) is the front view of the battery pack before the cover 41 is attached on the container 40, showing the internal structure of the battery pack. 42 in the figure denote the gas depolarizable button cells. The cell stack is held along the axial line so that the 6 cells are electrically connected in a series in the battery housing. 43, 44, and 45 are an electrolyte absorbed sheet, a hydrophobic filter, and a positive internal connector, respectively. One end of the connector is made of spring material that touches the positive terminal area of the cell located at the lowest part of the cell stack and that pushes up the whole cell stack with the spring pressure to achieve electrical contact. 46 indicates a negative internal connector, one end of which is connected to the negative terminal area of the cell located at the uppermost portion of the said cell stack. The other ends of the positive and negative internal connectors, 45 and 46, are fastened to the external positive terminal 47 and the negative terminal 48, respectively, with eyelet-like rivets 49. During discharge, oxygen in the air which is a cathode depolarizer enters into the battery housing through ventilators 50 and is supplied to each cell through the filter 44 and electrolyte absorbed sheet 43.
As shown in FIG. 6, a battery pack consisting of a plurality of gas depolarizable galvanic cells is packaged in a hermetically sealed bag and preserved until it is put into use. FIG. 6(A) depicts the way the battery pack 51 is inserted in the bag 52, while FIG. 6(B) describes the state of the bag 52 after the inlet of the bag is sealed and packaging of the battery pack 51 is completed. On the use of the battery, the battery pack is activated only when the bag is open and the battery is exposed to the air. Unlike the single cell application, the gas depolarizable galvanic cells which are composed of a cell stack are put in the battery housing without the sealing tape 39 shown in FIG. 4.
However, the conventional battery pack consisting of gas depolarizable galvanic cells have had many problems as follows.
(1) On high rate discharge, it has been observed that the diffusion rate of the air became low and the cathodic polarization occurred, leading to insufficient discharging, although notable problems have not occurred on low rate discharge.
(2) In some cases, zinc, the active material in the gelled anode in each cell, was oxidized and swelled in volume as discharge progressed. As a result, the electrolyte in the anode was squeezed and leaked through the air vent of the cell can. At the end of discharge, the cells having relatively small capacity in a battery pack consisting of more than three cells connected in a series were in an overdischarged condition accompanied by polarity reversal, followed by gas generation and severe leakage of the electrolyte in most cases. The existing battery packs have not been successful in complete stoppage of leakage, although the electrolyte absorbed sheet 43 and hydrophobic filter 44 were in use.
(3) Using the electrolyte absorbed sheet 43 and the hydrophobic filter 44 described above not only increases the number of parts, complicates the system, expands the man-hour of assembly processes, and increases the manufacturing cost, but it also causes the disadvantage of increasing the size of the battery pack.
(4) Although the battery packs are packaged in hermetic packaging bags and preserved before usage, the conventional sealing condition has not always been perfect.